Background/Question/Methods Because of the high concentrations of soil organic matter (SOM) in boreal forests, the response of this material to global warming will be an important driver of atmospheric CO2 concentrations.  To assess how heterotrophic microorganisms may alter their activities and thus their CO2-C return to the atmosphere with warming and the changing N availability likely to accompany warming, we collected soils from balsam fir forest floors and underlying mineral horizon (0~10cm) at the Main Brook and Codroy Valley sites along the NL Boreal Ecosystem Latitude Transect (NL-BELT) in western Newfoundland, Canada. We examined SOM dynamics at 15 ^oC and 20^oC during a 120-day laboratory incubation, with and without isotopically labeled coniferous litter additions. Litter additions with varying C:N ratios is to assess how organic N availability influenced the temperature response of microbial communities.  The heterotrophic respiration, δ¹³C of respired CO2-C and extracellular enzyme activity (EEA) were measured early and late in the incubation to assess microbial processing of C substrates with warming and altered N availability, via the distinct δ¹³C of the supplemental litter. Results/Conclusions

Soil warming increased CO2 release from all soils as expected, particularly with litter additions; CO2 losses were greatest in Main Brook soils, suggesting greater potential for SOM loss with warming at this more northern site. The changes in δ¹³C of respired CO2-C indicated significant respiration of added litter at the early stage of the incubation. Warming resulted in all soils releasing relatively ¹³C-enriched CO₂ early in the incubation, indicative of a temperature effect on microbial substrate choice. Soil microbial biomass rapidly increased during the early stage, then significantly decreased later, suggesting resource limitation on microbial activity during the late stages of these incubations. Lack of differences in microbial biomass among litter and litter N content suggest that this limitation was not substrate or N induced. The influence of temperature on these processes was generally more significant than that of litter addition quality. Early in the incubation, we observed a decrease in enzyme activity associated with microbial acquisition of labile SOM (PHOS, BG, and LAP) with warming, and a corresponding increase in oxidative enzyme activity, particularly in the Main Brook soils. These data suggest warming can cause a shift in microbial use of SOM, and potentially increase the use of more recalcitrant SOM. Further, the more northern soils along this transect appear more sensitive to this change. The study can help address how temperature influences the microbially-mediated SOM transformations.